iv ENVIRONMENT AND LIFE 181 



ment illustrates from the very first a fact which is 

 more or less familiar to all, the fact that living organisms 

 can withstand, generally speaking, but a small amount 

 of environmental modification. They are in so many 

 ways, and by so many parts, dependent upon the ex- 

 ternal medium, their adaptation to it is so very close, 

 and the slightest change in environment is apt to react 

 on such a large proportion of the vital functions, that 

 we cannot wonder at the enormous influence which 

 external modifications can exert on life. Suppose 

 for instance the very small percentage 0,030-0,034% of 

 carbonic acid which always exists in our atmosphere, 

 were to disappear, life would soon be extinct on the 

 whole earth, because plants cannot do without it, nor 

 animals without plants. Thus a very small change, 

 which would be perceived only through the use of precise 

 methods and instruments, and could not be detected by 

 our unaided senses, would suffice to ruin all life. This 

 instance shows how very close is the adaptation 

 between organisms and their environment, and 

 teaches the need of care in all our experiments on 

 the action of the latter on the former. A great 

 number of instances are known which show the 

 considerable influence of minute external variations, 

 but none, I think, is more cogent than that which I 

 gather from the excellent Etudes chimiques sur la 

 Vegetation of Jules Raulin (Paris, 1870). This writer, 



